In Patent Documents 1 and 2, for example, expansion actuators are proposed which control the expansion and contraction of the upper link and the lower link of the vehicular suspension system so as to enhance steering stability performance by suppressing changes in the camber angle or the contact patch accompanying bumping and rebounding of the wheel. Such an expansion actuator is configured by an electric motor and a feed screw mechanism having an external thread member and an internal thread member which are rotated relative to each other by the rotation of a rotating shaft of this electric motor. The rotating shaft, i.e., an output member, of the electric motor and the rotating shaft, i.e., an input member, of the feed screw mechanism are coupled to each other by a shaft coupling mechanism (coupling), and this shaft coupling mechanism has two outer bushings formed of polyacetal and one inner bushing formed of silicone rubber, each of these bushings being provided with a plurality of projections projecting radially at equal intervals from an outer periphery thereof. The rotating shaft of the electric motor and the rotating shaft of the feed screw mechanism are connected to the respective bushings by means of respective coupling base bodies provided with a plurality of axially extending projections which are disposed between projections of the respective bushings.